When you create something, you already have the copyright, but you can add the Creative Commons license layer to that work. You are not giving up your copyright, but specifically modifying your copyright to indicate how that content can be used by others. As Educause states:

Higher education is rooted in the belief that the free exchange of knowledge is fundamental to the common good, and faculty and researchers in large numbers have begun using Creative Commons licenses to facilitate a climate of openness and sharing.